This invention relates to a method for inhibiting oxidation of a reduced metal article. More particularly, the invention relates to a method of inhibiting rusting of sponge iron.
Sponge iron, metallized pellets, briquettes, or reduced metal materials are produced by the direct reduction of ores. Large quantities of metallized iron pellets are made in the direct reduction process wherein particulate iron ore is reduced substantially to metallic iron by direct contact with a reducing gas such as a mixture of hydrogen and carbon monoxide. Throughout this specification and the appended claims, the term "metallized pellets" is intended to include iron bearing pellets such as sponge iron, briquettes, other compacted forms of reduced iron and the like which contain at least 80% of their iron in the metallic state with the balance being virtually all in the form of iron oxide. "Metallized," in this sense, does not mean coated with metal, but means nearly completely reduced to the metallic state. This metallized iron product is suitable for charging directly to a steelmaking furnace such as an electric arc furnace, as a feed material. One of the problems associated with the use of sponge iron as a raw material in steelmaking is its inherent tendency to reoxidize upon exposure to air or water. Hot sponge iron is extremely reactive because of its high surface area and high porosity, and oxidizes spontaneously if contacted by oxygen in any form. Thus sponge iron must be cooled in a reducing or neutral atmosphere. Sponge iron oxidizes even when stored in the open air. Iron oxidizes in dry air to form magnetite by the well-known reaction: EQU 3Fe + 2 O.sub.2 .fwdarw. Fe.sub.3 O.sub.4 ( 1)
this oxidation reaction is very exothermic which can cause a detrimental loss in metallic iron content or metallization, as well as cause a bulk mass of sponge iron to heat up excessively and burn. When water contacts iron in the presence of atmospheric oxygen, rusting occurs in two stages.
The first stage rust, ferrous hydroxide, is formed by the reaction: EQU Fe + H.sub.2 O + 1/2 O.sub.2 = Fe(OH).sub.2 ( 2)
the second and final stage rust, hydrated ferric oxide, is formed by the reaction: EQU 2Fe(OH).sub.2 + H.sub.2 O + 1/2 O.sub.2 = Fe.sub.2 O.sub.3 . 3H.sub.2 O (3)
a third reaction takes place which liberates hydrogen. Part of the first stage rust recomposes to form the iron oxide known as magnetite, water, and hydrogen by the reaction: EQU 3Fe(OH).sub.2 = Fe.sub.3 O.sub.4 + 2H.sub.2 O + H.sub.2 ( 4)
reactions (2) and (3) are also exothermic. The amount of heat liberated by reacting only approximately 15% of the initial water is capable of supplying the latent heat of evaporation of the remaining 85% of the initial water. Although reaction (4) is slightly endothermic, the cooling effect is negligible. Exposure of a mass of active sponge iron to atmospheric air and moisture will cause rusting with a significant loss of metallization. Such exposure will also produce heat and can raise the temperature of the mass so high that the liberated hydrogen ignites.
Uses of directly reduced sponge iron products are restricted unless such products can be stored in bulk in the open, subject to contact by the elements.
A potential problem is created by transportation of sponge iron in bulk, as in barges or closed containers, such as by ship. Water can enter the holds of most ships wetting the loosely stored material therein. When the material is untreated sponge iron, the water reacts with it as described above, releasing large quantities of hydrogen. Special ventilation procedures are required in order to prevent the buildup of hydrogen in the hold to combustible proportions and minimize the risk of such shipments by water.
Numerous methods have been proposed to prevent reoxidation of sponge iron and to prevent such hydrogen generation. One suggested method is to eliminate the large surface areas by making relatively nonporous aggregates. This is expensive and requires heavy compaction equipment. Another method is to coat the particles with hydrocarbons such as asphalt, plastics, or waxes. These coatings are expensive, difficult to apply, and contaminate the product. Such coatings are easily damaged; for instance, a mere shift of material in its container during transit may rupture the coating thus exposing active iron surfaces to the atmosphere. In such cases, when a fire breaks out, the hydrocarbon coatings actually add to the intensity of the flames. U.S. Pat No. 3,844,764 discloses an effective process for passivation against reoxidation of sponge iron in air in the dry state. However, this process does not prevent rusting when the passivated sponge iron is wetted with water and exposed to air.
It has now been found that by treating sponge iron with a solution of a water soluble alkali metal silicate, such as sodium silicate, the product can be stored for long periods of time in the open, even in rainy weather, or be shipped in open trucks and railroad cars without any significant rusting or loss of metallization. A large mass of the product can be shipped over great distances in a wet hold with such minimal hydrogen generation that shipboard ventilation systems can easily maintain the hydrogen generated well within safe limits. The process which inhibits reoxidation of sponge iron in air, as well as against rusting when wetted with water, consists of wetting the sponge iron with a dilute aqueous solution of liquid alkali metal silicate, preferably followed by drying the wetted sponge iron. The drying step is preferably carried out under oxidizing conditions and at a temperature substantially below the autoignition temperature of sponge iron. During the wetting and drying process, there is no measurable loss in metallization, and when later rewetted with water, there is no measurable rusting or heating. If the drying step is omitted, and the sponge is simply wetted with a dilute aqueous solution of liquid alkali metal silicate, there is no measurable rusting or heating while it is wet. It has been discovered that an aqueous solution of liquid alkali metal silicate is an unexpectedly and exceedingly effective rust inhibitor for sponge iron. Alkali metal silicates are readily removed from sponge iron into slag during the operation of a steelmaking process, and are thus only minimal contaminants.